A variety of techniques exist in the field of orthopaedic surgery for treating bone fractures. Many known techniques utilize bone screws and bone fixation plates. Typically, the plate is used to stabilize the site of a bone fracture, and one or more bone screws are inserted through apertures of the plate and threaded into the bone material so as to secure the plate to the bone material. It is also known that bone screw/plate systems can be improved by machining a thread onto the head of the bone screw, in addition to the thread normally machined onto the main shaft of the screw. In connection with the use of threaded-head screws, the apertures of the plate are threaded to matingly receive the threads of the screw head. Thus, as the screw is inserted into an aperture of the plate and threaded into the bone material, the head of the screw likewise is threaded into the aperture. As a result, the screw becomes rigidly affixed to the plate, in effect locking to the plate rather than simply bearing against the plate. Examples of threaded-head bone screws and threaded-aperture bone plates are disclosed in U.S. Pat. No. 5,709,686 to Talus et al.: U.S. Pat. No. 6,206,881 to Frigg et al.: and U.S. Pat. No. 6,306,140 to Siddigui.
The use of threaded-head screws and threaded-aperture plates provides certain advantages. It is known that some types of small bone fragments tend to change position relative to the plate over time. This deleterious condition can result from the “toggling” of the screws affixed to the plate. However, when multiple screws are rigidly fixed to the plate by mating the respective threads of the screw heads with the threads of the corresponding plate apertures, the screws do not toggle in the plate. The locking action provided by the threaded-head screw in combination with the threaded-aperture plate prevents motion between the bone fragment and the plate as well as premature loosening of the screws.
Although the use of threaded-head screws has provided improvements in orthopaedic surgical techniques, there remains the disadvantage that these screw/plate systems are unidirectional. That is, the thread formed on the inside surface of the aperture of the plate is structurally fixed at a constant helical angle with respect to the central axis passing through the center point of the aperture. Hence, the head of a conventional threaded-head screw can only be rigidly affixed to the plate by mating its thread with that of the aperture, such that the bone screw is always inserted and threaded in one direction, e.g., perpendicularly or coaxially with respect to the plate.
Recent developments in this field provide screw/plate systems that allow the surgeon to choose the angle at which the screw is inserted through, and rigidly affixed in, an aperture of the plate. Examples of such systems are disclosed in U.S. Pat. No. 6,955,677 to Dahners. Such improvements enable the surgeon to direct the bone screw toward bone fragments that are not situated directly beneath the aperture of the plate, and also provide flexibility in the placement of the plate in relation to the bone fracture. These systems, however, do not address the possibility of the screws unscrewing or backing out of the plate due to common loading and unloading associated with this application. This is problematic if it should occur since the plate becomes loose and fails to perform its intended function properly. Furthermore, in anatomically critical areas, such as the anterior cervical spine, impingement of the backed-out screw on overlying structures can even cause significant morbidity and mortality.
Also, current screw driver-like driver tools for inserting fasteners such as screws are limiting in that angular placement is difficult due to the tendency of the screws to fall off the driver tool when placed in an angular orientation.
It would therefore be advantageous to provide a further improved screw/plate system that prevents the screw from backing out of the plate regardless of whether the screw is in a coaxial or non-coaxial position relative to the aperture in the plate. Although the screw cannot back-out of the plate, the system can further include a driving tool that permits the surgeon to secure and remove the screw if necessary in order to remove and recontour or resituate the plate, to adjust the screw angle if the initial insertion proves unsatisfactory, or to remove the plate after fracture healing. Thus, the further improved system provides anti-unscrewing capabilities, while at the same time offering selective removability. Furthermore, a driver tool is provided that can facilitate use of such a system and can provide improved angular adjustability and control for inserting screws at various angles to the central axis of an aperture in the plate.